When a short-circuit fault occurs in a power system, a short-circuit current flows from a power generator connected to the system to a short-circuit point. A value obtained by multiplying the short-circuit current by a line-to-line voltage is a short-circuit capacity, and the short-circuit capacity tends to increase in recent power systems. This is because a large-scale power source is non-uniformly located in a basic system, and introduction of a distributed power source is advancing in the lower-level system.
When the short-circuit capacity of the power system increases, the short-circuit current flowing at the time of a system failure increases, and may exceed the rated breaking capacity. In this case, the co-existing breaker may be replaced with a higher-level rated breaker, but it brings about an increase of costs. Hence, technologies are proposed which employ a high-impedance device and a current-limiting reactor, and which divide the system to suppress a short-circuit capacity of the power system.
Among those technologies, a division of a system is quite effective to suppress a short-circuit capacity. More specifically, a scheme of always dividing the system, a scheme of introducing a new high-order system voltage to divide the co-existing system, and a scheme of dividing an AC system by a DC interconnection (BTB: Back To Back) are known so far.
However, when a divisional operation of the system is carried out to suppress the short-circuit capacity of the power system, the system operation becomes inevitably non-flexible, and thus the advantages of a system interconnection may be lost. That is, in order to maintain the flexibility of the system operation, it is desirable that the divisional operation of the system should be suppressed as minimum as possible.
In order to do so, it is necessary to precisely know the short-circuit capacity. When the precise short-circuit capacity is known, a flexible system operation and a selection of a set value of a protection relay in accordance with an actual condition are enabled. That is, from the standpoint of a system protection, it is important to know the short-circuit capacity.
However, the short-circuit capacity of a power system has the magnitude and the distribution continuously changing depending on various factors, such as a condition of a system configuration when a short-circuit failure occurs (e.g., a system switching in a higher-rank system), the number of power generators connected in parallel, the location of the short-circuit point, and the kind of the failure.
It is difficult to directly measure the short-circuit capacity of the power system. Hence, according to the conventional technologies, the short-circuit capacity is calculated based on preset constants of system facilities, such as a power line constituting the system, a transformer, and a power generator (see, for example, Non-patent Document 1). When, however, the constants of system facilities are applied, it is necessary to perform calculation with all power-generator parallel connection condition and system configurations being reflected. Hence, it requires a lot of works and time. In addition, the value obtained through the calculation contains unclarity in the precondition of the calculation and the constants. As a measurement scheme of the short-circuit capacity, an indirect power-system short-circuit capacity measuring scheme is general. For example, the short-circuit capacity is indirectly obtained based on the measured value of a voltage fluctuation rate inherent to a loading of a power capacitor or a shunt reactor (see Non-patent Document 2).